1. Field of the Invention
The present invention relates to a laser beam transmission apparatus which transmits a laser beam with good convergence properties.
2. Description of the Related Art
FIG. 6 is a view showing the structure of a laser processing apparatus. A solid-state laser apparatus 1 is of a rod-type. The solid-state laser apparatus 1 has a total-reflection mirror 3 at one end of a laser resonator 2, and a partial-reflection mirror 4 at the other end. The total-reflection mirror 3 and partial-reflection mirror 4 are disposed opposed to each other.
Two laser rods 5 and 6, for example, are provided between the total-reflection mirror 3 and partial-reflection mirror 4 in series on a laser optical axis.
The solid-state laser apparatus 1 has an excitation section (not shown) that excites the two laser rods 5 and 6.
With this structure, when the two laser rods 5 and 6 are excited, laser resonance takes place between the total-reflection mirror 3 and partial-reflection mirror 4. The laser resonance gradually increases the laser beam energy. When the laser beam energy has reached a predetermined value or more, a laser beam is emitted from the partial-reflection mirror 4.
A converging lens 7 is provided on an optical path of the laser beam emitted from the solid-state laser apparatus 1. The converging lens 7 converges the laser beam output from the solid-state laser apparatus 1 and makes it incident on a light-incidence end portion 9 of an optical fiber 8. The converging lens 7 makes the laser beam incident on the optical fiber 8.
It is common art to use the converging lens 7 for making the laser beam incident optical fiber 8. For instance, Jpn. Pat. Appln. KOKAI Publication No. 8-167754 and Jpn. Pat. Appln. KOKAI Publication No. 7-307513 disclose techniques wherein a laser beam is made incident on an optical fiber (8) using a converging lens group.
The optical fiber 9 is laid between the location of the solid-state laser apparatus 1 and a place for processing work. A light-emission end portion 11 is provided at the other end of the optical fiber 9.
The optical fiber 8 guides the laser beam, which has entered from the light-incidence end portion 9, and emits it from the light-emission end portion 11. The light-emission end portion 11 is provided with a head lens 12 that constitutes a processing head.
Thus, the laser beam emitted from the light-emission end portion 11 is converged through the head lens 12 and applied to a workpiece 10. The workpiece 10, for example, is welded or cut by the application of the laser beam.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2001-94177 also discloses a technique for making a laser beam incident on an optical fiber (8). FIG. 7 shows the structure of a light-incidence optical system disclosed in this document.
A first lens 13 and a second lens 14 are provided in series on a laser beam optical axis between a solid-state laser apparatus 1 and a light-incidence end portion 9 of an optical fiber 8. The first lens 13 and second lens 14 constitute a telecentric optical system.
The focal distance of the first lens 13 is f1, and that of the second lens 14 is f2.
The distance between a light-emission plane F of solid-state laser apparatus 1 and the first lens 13 is set at f1, and the distance between the first and second lenses 13 and 14 is set at f1+f2.
The distance between the second lens 14 and a light-incidence plane R of optical fiber 8 is set at f2.
The beam emission diameter of the laser beam emitted from the light-emission plane F is D1.
The beam incidence diameter of the laser beam incident on the light-incidence plane R is D2.
According to the following equation, the light-incidence optical system focuses the laser beam with the beam emission diameter D1 on the light-incidence plane R:D2=(f2/f1)D1  (1)
Accordingly, the light-incidence optical system reduces the beam emission diameter D1 to the beam incidence diameter D2, which is equal to a value obtained by multiplying D1 by (f2/f1).
However, in the techniques of Jpn. Pat. Appln. KOKAI Publication No. 8-167754 and Jpn. Pat. Appln. KOKAI Publication No. 7-307513, if the divergence angle of the laser beam emitted from the laser apparatus 1 varies, a minimum spot position of the converging lens 7 shifts in the laser optical axis direction.
When the laser processing apparatus, for example, welds the workpiece 10, it increases or decreases the laser output. In general, when the laser output of the rod-type solid-state laser apparatus 1 is increased or decreased, the emission beam diameter D1 of the laser beam and the divergence angle of the beam will vary.
Specifically, FIG. 8 shows the emission beam diameter and the divergence angle of the beam when the laser output is decreased. FIG. 9 shows the emission beam diameter D1 and the divergence angle φ of the beam when the laser output is decreased.
The divergence angle φ of the beam tends to increase in accordance with an increase in laser output. The emission beam diameter D1 tends to decrease in accordance with an increase in laser output.
When the laser output is small, the minimum spot position is present at a distance f, as shown in FIG. 8. However, if the laser output is increased, the minimum spot position shifts farther by a distance g from the converging lens 7, as shown in FIG. 9. As a result, the minimum spot position shifts to a point of distance f+g from the converging lens 7.
The optical fiber 8 comprises a core layer and a clad layer that are coaxially arranged. It is required that the laser beam be converged to have a beam incidence diameter D2 that is substantially equal to a core diameter and then enter the core layer.
However, if the minimum spot position shifts, the beam incidence diameter D2 does not agree with the core diameter. To solve this problem, the core diameter has to be set at a sufficiently large value, considering in advance the beam incidence diameter D2 at a time the laser output may be increased.
However, if the laser beam is transmitted through the optical fiber 8 with an increased diameter, the beam quality of the laser beam cannot be maintained, and the optimal beam quality cannot be achieved.
On the other hand, in the telecentric optical system shown in FIG. 7, the beam with the beam emission diameter D1 is focused with a reduced diameter.
Thus, an optical fiber with a small core diameter can be used. Thereby, the telecentric optical system can maintain the beam quality of the laser beam and achieve a maximum beam quality.
However, in order to make the laser beam incident on the optical fiber with a reduced diameter, it is necessary to make the numerical aperture NA (=sin α) of the laser beam closer to a value permissible by the optical fiber 8, as shown in FIG. 10.
In general, the intensity of a laser beam exhibits a Gaussian distribution. In order to transmit a laser beam without causing damage to the optical fiber 8, it is necessary to make the beam incidence diameter D2 sufficiently small, relative to the core diameter of the optical fiber 8.
However, the intensity of a laser beam exhibits a Gaussian distribution and it is necessary to make the numerical aperture NA (=sin α) of the laser beam closer to a value permissible by the optical fiber 8. Therefore, it is not possible to make the beam incidence diameter D2 sufficiently small, relative to the core diameter of the optical fiber 8.
As mentioned above, the emission beam diameter D1 of the solid-state laser apparatus 1 decreases as the laser output increases. On the contrary, the emission beam divergence angle φ tends to increase. Thus, when the light-incidence optical system shown in FIG. 7 is to be designed, it has to be considered that the laser beam can enter the optical fiber 8 with safety in a region of a low laser output.
However, the beam incidence diameter D2 becomes smaller than the core diameter of optical fiber 8 in a high output region where the emission beam diameter D1 is small. Thus, the beam quality of the laser beam cannot fully be exhibited.
In the light-incidence optical system shown in FIG. 7, the distance between the light emission end of solid-state laser apparatus 1 and the first lens 13 is set at f1, and the distance between the second lens 14 and the light-incidence end portion 9 of the optical fiber 8 is set at f2.
Normally, the core diameter of the optical fiber 8 is small and, e.g. about 1/10 to 1/20 of the emission beam diameter D1 of the laser beam emitted from the solid-state laser apparatus 1. It is thus necessary to reduce the emission beam diameter D1 with a nearly equal focusing magnification.
As expressed by the above equation (1), the focusing magnification is determined by a ratio (f2/f1) between the focal distance f1 of first lens 13 and the focal distance f2 of second lens 14. In order to decrease the beam emission diameter D1, it is thus necessary to increase the focal distance f1 of first lens 13.
As a result, the distance between the light-emission end portion of the solid-state laser apparatus 1 and the light-incidence end portion 9 of the optical fiber 8 increases, and the size of the whole laser processing apparatus increases.
Moreover, the distance f1 between the light-emission plane F of solid-state laser apparatus 1 and the first lens 13 and the distance f2 between the second lens 14 and the light-incidence plane R of optical fiber 8 are determined. Thus, the degree of freedom in design is limited as regards the variation in the distance between the light-emission plane F of solid-state laser apparatus 1 and the light-incidence plane R of optical fiber 8.